1. Field of the Invention
The present invention relates to an image processing circuit, and a display panel driver and display device mounting the image processing circuit.
2. Description of Related Art
In a mobile apparatus such as a mobile phone and a PDA (Personal Digital Assistant), an LCD (Liquid Crystal Display) panel or another display panel is generally mounted. Since the mobile apparatus relies on a battery for a power supply, it is important to reduce power consumption of a display panel and a display panel driver for driving the display panel (for example, an LCD driver), resulting in extension of an operating time.
One method for reducing power consumption is to mount an image memory on the display panel driver and reduce the frequency of access to the image memory. For example, if image data is written to the image memory only in a case where an image is changed, an electric power required for transfer of image data is reduced and accordingly the power consumption can be reduced.
One problem about the mounting of the image memory on the display panel driver is increase in a required memory capacity. In these years, resolution and the number of gradations of the display panel are increasing because of diversification of contents to be displayed. For this reason, increase in a capacity of the image memory is demanded. However, since leading to increase in a cost, the increase in the capacity of the image memory is not favorable.
One method for reducing the memory capacity is to compress image data and store the compressed data in the image memory. Various types of compression methods of image data stored in the image memory are proposed.
One commonly-known compression method is block coding for performing a compression process in units of blocks including a plurality of pixels. In the block coding, image data of a plurality of pixels constituting the block is represented by at least one representative value. For example, Japanese Laid-Open Patent Application JP-P 2007-312126A (US2007269118 (A1)) discloses a compression method for representing image dada of pixels in a block by a plurality of representative value. In the compression method described in this publication, one of three-level BTC (Block Truncation Coding) and two-level BTC is selected depending on image data, and compression of the image data is performed by the selected coding technique. Specifically, RGB data is converted into YUV data, and the three-level BTC is employed in a case where a difference in brightness data of pixels and a difference of color difference data are large in each block. In a case where the differences are not so large, the two-level BTC is employed. In addition, Japanese Laid-Open Patent Application JP-A-Heisei 10-66072 discloses a compression method for constituting compressed image data by using an average value, a deviation, and a piece of bit plane information of image data in pixels of a block.
One problem of the block coding is block noise caused by a difference of correlation between adjacent blocks. For example, when a compression process is performed in units of blocks each including 4 pixels, a case will be considered in which: a correlation of image data among 4 pixels in a certain block is high; and a correlation of image data among 4 pixels in an adjacent block is low. In this case, a large error exists in the block with low correlation, and the block with a large error is arranged next to the block with a small error. Human eyes will recognize this as block noise.
Another commonly-known compression method is a method for independently processing image data of each pixel such as the dither processing using a dither matrix. Such compression method is disclosed in, for example, Japanese Laid-Open Patent Application JP-P 2003-162272 A (U.S. Pat. No. 7,483,574 (B2)). In the compression method for independently processing image data of each pixel, block noise is not generated. However, in the compression method for independently processing image data of each pixel, there is a problem that generates granular noise in an image in which pixels with high correlation of image data are arranged.
Japanese Laid-Open Patent Application JP-P 2006-311474A (US2006220984 (A1)) discloses a technique that: uses the block coding for an image with moderate gradation; and independently processes image data of each pixel when gradations of adjacent pixels are widely different from each other. This publication describes that the two functions are necessary to perform the image processing for any images without collapse.
However, the inventors have now discovered the following facts. According to study by the inventors, the compression method disclosed in Japanese Laid-Open Patent Application JP-P 2006-311474A does not substantially reduce granular noise. When even one pixel having low correlation of image data to other pixels exists in pixels included in a block, the technique disclosed in Japanese Laid-Open Patent Application JP-P 2006-311474A employs a compression method for independently processing image data of each pixel. For example, in a case where the block includes pixels arranged in 2 rows by 2 columns, even when a correlation of image data between two pixels is high and a correlation of image data of remaining two pixels with the former two pixels is low, a compression method for independently processing image data of each pixel is employed. In this case, the compression method for independently processing image data of each pixel is employed for the pixel with a high correlation of image data, resulting in generation of granular noise.